Some conventional condenser microphone are of a stick type including replaceable microphone units of different directivity characteristics (see, for example, Japanese Patent Nos. 4533784, and 4683996). Such a microphone device is provided with, for example, cardioid, hyper cardioid, nondirectional, and bidirectional microphone units, from which a user selects one suitable for the purpose and mounts the selected one to a microphone body having a built-in power module.
The microphone device including replaceable microphone units includes a unit case for accommodating one of the microphone units, a circuit case accommodating a signal processor for processing an audio signal output from the microphone unit, a cap for covering an opening of the unit case, and a rear case accommodating a connector for outputting signals generated by the signal processor to an external device.
FIG. 7 is a cross-sectional front view illustrating components accommodated in a unit case of a conventional microphone device. FIG. 7 illustrates a cap 2 is fixed on a unit case 3. Now, an example of the microphone device to which a unidirectional microphone unit is mounted is described.
A microphone unit 31 is accommodated in the front side (upper side in the drawing) in the unit case 3. An inner plate 34 is fitted and fixed in the rear side (lower side in the drawing) in the unit case 3. An outer plate 35 is fitted and fixed to the rear side of the inner plate 34, in the unit case 3. A first terminal 37, which is a conductive terminal having a rim, is inserted into a hole formed on the center of the outer plate 35 from the inside (front side) toward the outside (rear side) of the unit case 3. A rod shaped terminal 33 is inserted into a hole formed on the center of the inner plate 34 from the outside toward the inside. The terminal 33 is fixed to the inner plate 34 so as to project toward the microphone unit 31. A conductive spring 36 is disposed in the central holes of the inner plate 34 and the outer plate 35. The first terminal 37 has a semispherical shaped rear end. The first terminal 37 is accommodated in the unit case 3 and protrudes outwardly from the outer plate 35 by the urging force of the spring 36. Thus, the rear end of the first terminal 37 is projecting outwardly from the outer plate 35. An output terminal plate 32 extending from the microphone unit 31 is screwed to the front end of the terminal 33.
FIG. 8 is a cross-sectional front view illustrating components accommodated in the circuit case and components accommodated in the rear case attached to the circuit case of the conventional microphone device. A front end plate 41 is fitted and fixed in the inner circumference of the circuit case 4 at the top adjacent to an internal thread 4S and on the rear of the internal thread 4S. A conductive second terminal 42 penetrates the center of the front end plate 41 across the thickness (longitudinal direction in the drawing) and is fitted and fixed to the front end plate 41.
A circuit board 43 is disposed in the circuit case 4 and along the length direction of the circuit case 4. The circuit board 43 constitutes a signal processor that processes signals from the microphone unit 31. The second terminal 42 is connected to an input terminal of the circuit board 43 with an appropriate conductive material provided therebetween.
A rear case 5 is fitted to the rear end of the circuit case 4, and the circuit case 4 and the rear case 5 are fastened with a screw 6. The rear end of the circuit board 43 is received in a deep groove 51 in the rear case 5. The circuit board 43 is electrically connected to the rear case 5.
In the rear case 5, an insulation connector base 52 is fitted and fastened, for example, with a screw. The connector base 52 is provided with a connector having three standardized conductive pins. The connector is embedded in the connector base 52. A first pin 53 is connected to a ground terminal of the circuit board 43. A second pin 54 is connected to an output terminal at a hot side of an output circuit of the circuit board 43. A third pin (not illustrated) is connected to an output terminal at a cold side of the output circuit of the circuit board 43. The rear case 5 has a groove 55 for positioning in the rotating-direction on the connector base 52.
A connector plug (not illustrated) is inserted into the rear case 5. Audio signals from the microphone unit 31 are output to an external device via the second pin 54, the third pin, and a microphone code connected to a connector plug.
FIG. 9 is a cross-sectional front view illustrating the conventional microphone device. An external thread 3S of the unit case 3 is screwed into the internal thread 4S of the circuit case 4. As a result, the spring 36 is compressed. The unit case 3 is connected (attached) to the circuit case 4. After the unit case 3 and the circuit case 4 are attached to each other, the first terminal 37 of the unit case 3 is pressed by a compressive force of spring 36. The first terminal 37 of the unit case 3 comes in contact with the second terminal 42 of the circuit case 4. As a result, the first terminal 37 of the unit case 3 is electrically connected with the second terminal 42 of the circuit case 4.
Audio signals from the microphone unit 31 are sent to an input terminal of the circuit board 43 via the output terminal plate 32, the terminal 33, the spring 36, the first terminal 37, and the second terminal 42. The circuit board 43 performs predetermined processes such as impedance transformation of the audio signals received from the microphone unit 31, low-cut processing, and amplification to generate output signals. The output signals generated in the circuit board 43 are transmitted to the first pin 53, the second pin, 54, and the third pin.
An impedance of a signal source is a capacitance (of approximately several pF to several tens of pF). Thus, the condenser microphone requires an impedance transducer having a field-effect transistor (FET). The FET includes a gate electrode, a drain electrode, and a source electrode. The signals from the microphone unit are sent to the gate electrode. The FET may break if high-voltage electric current flows into the gate electrode. To avoid such an FET breakdown, a protective diode is connected to an input for the signals from the microphone unit.
Unfortunately, the method of connecting the protective diode to prevent the FET breakdown may increase the stray capacitance of the input due to the electrostatic capacitance of the diode, and thus decrease the sensitivity of the microphone device.
An electrostatic shield is formed by the unit case 3 and the circuit case 4 after the unit case 3 and the circuit case 4 are attached to each other. This prevents the gate electrode from being affected by high voltage. In contrast, the electrostatic shield is not formed after the unit case 3 and the circuit case 4 are detached for replacement of the microphone unit. In such a state, if a charged human body touches a component provided for the connection with the unit case 3 accommodated in the circuit case 4, for example, the second terminal 42, the electric charge of the human body flows into the gate electrode via the connection component, and this breaks the FET.
In another case, if the unit case 3 and the circuit case 4 are detached during the electrical operation of the components in the circuit case 4, the microphone device produces loud noise due to disconnection between the contact points of the unit case 3 and the circuit case 4.
As described above, the gate electrode is electrically disconnected from the connected components if the unit case 3 and the circuit case 4 are detached. The FET breakdown or the noise production can be prevented, however, by grounding the connection component if the unit case 3 and the circuit case 4 are detached.